Asphaltenes are present in crude oils. Asphaltenes include a large number of structures, such as high molecular weight fused aromatic compounds with heteroatoms. Asphaltenes are heterocyclic unsaturated macromolecules primarily of carbon and hydrogen, but also containing minor components such as sulphur, oxygen, nitrogen, and various heavy metals. Due to the complexity of their chemistry, asphaltenes can be defined as the fraction of crude oil which is insoluble in heptane.
In crude oil, asphaltenes are usually present as a colloidal dispersion which is stabilised by oil resins. Under normal reservoir conditions, asphaltenes are in equilibrium in the crude oil. As crude oil is produced, the equilibrium may be altered by a number of factors, such as carbon dioxide injection, pH change, pressure change, temperature change, shear or streaming potential through porous media resulting in asphaltene flocculation or precipitation, and asphaltene deposition onto surfaces.
Asphaltene precipitation and/or deposition can occur anywhere in the crude oil production life cycle from the reservoir to the refinery. For example, asphaltene deposition may occur in the near-wellbore region including perforations, in the tubing, downhole and surface chokes, and surface flowlines. Predicting where asphaltene deposition might occur requires an understanding of the mechanisms for asphaltene deposition. The main causes are pressure decrease and injection of incompatible fluids in the reservoir rocks. Asphaltene deposition can significantly reduce well productivity, causing operational problems during production and processing of crude oils, damaging formations, and decreasing production.
Asphaltenes may also precipitate out and deposit during production, refining, transportation and storage of any products derived from crude oil, e.g. heavy heating oil or marine oil.
Heavy oils, which are often used for powering ships, comprise considerable amounts of asphaltenes. Precipitating out of asphaltenes can lead both to poor combustion and to difficulties during handling and storage of the fuel. Combustion disturbances due to precipitating out of asphaltenes are also observed in power stations operated with heavy oils.
Some oils comprise hydrocarbon waxes which precipitate out at low temperatures. Interactions between the precipitating out of wax and asphaltenes can increase the total amount of substance precipitated out and/or the rate of formation thereof.
Asphaltenes can be deposited in valves, pipes and other process equipment, for example in an oil refinery. On hot surfaces, for example heat exchangers, carbonization of these deposits can make their removal very difficult. The deposits reduce the efficiency of process equipment and in the worst case scenario can result in a complete blockage and stop in production, which results in high costs.
Asphaltene deposits can be removed by using solvents such as xylene or xylene mixtures which typically have undesirable health, safety, and environmental concerns. In general, prevention of asphaltene precipitation is more effective than attempting the removal of the precipitates or deposits. Precipitating out of asphaltenes can be reduced by adding a dispersing agent to the oil. If precipitates of asphaltenes have already formed, the addition of a dispersing agent can improve the effectiveness of any solvents used to remove the deposits.
Currently, several major chemical classes of asphaltene dispersants are commercially available and used in the oil industry. Examples of patent specifications describing the use of different asphaltene dispersants include:
U.S. Pat. No. 4,414,035 discloses the use of dodecylbenzene sulfonic acid (DDBSA) as an asphaltene dispersant. DDBSA is widely used in the industry for its effectiveness and low cost. However, there are environmental concerns about the use of DDBSA.
U.S. Pat. No. 5,925,233 describes the use of alkanesulfonic acid as an asphaltene dispersant. It can be used alone, or in formulation with alkyl-formaldehyde resin, oxyalkylated amines, or wax-dispersing agents.
U.S. Pat. No. 6,313,367 describes the use of esters derived from polyhydric alcohols and carboxylic acids (such as sorbitan mono-oleates) as asphaltene dispersants.
US2011/0092393 discloses the use of hydrophobically modified non-ionic polysaccharides as viscosity reducers in petroleum applications, such as recovery and transport of heavy petroleum oils. The polysaccharides are modified with, for sample, hydrophobically end-capped 2-hydroxyethyl groups.
US2012/0220807 describes polymer asphaltene dispersants derived from the polymerisation of α-olefin and vinyl pyrrolidinone.
US2012/0004344 discloses the use of acrylate copolymers of 2-ethylhexyl methacrylate and n-butyl acrylate as an asphaltene drag reducing additive for crude oil.
WO2016/053634 describes asphaltene dispersants derived from the reaction product of an alk(en)yl substituted succinic anhydride such as polyisobutylene succinic anhydride (PIBSA) and a polyol.
Despite the wide number of materials disclosed as having asphaltene dispersancy properties, there is still a need for environmentally-friendly asphaltene dispersants for crude oil which have improved properties over existing materials such as reducing (i) the amount of asphaltene precipitate formed, (ii) the speed of precipitate formation, (iii) the particle size of the precipitate, (iv) the tendency of the precipitate to be deposited on surfaces, and/or (v) the amount of asphaltene dispersant required to be effective.